In U.K. patent specifications Nos. 1222508 and 1475881, a process has been described for removing impurities from clay and particularly china clay. In that process a suspension of the clay in water is mixed with a non-polar organic liquid and a collector which gives the impurities a hydrophobic surface. On settling, the impurities collect in an interfacial layer between the water of the suspension and the organic liquid.
The problem associated with the process of the above mentioned specifications lies in the separation of the impurities-containing interfacial layer from both the aqueous phase and the organic phase. Specification No. 1475881 has proposed a process line entailing mixers and settlers and a centrifuge, but that line, besides being expensive, is difficult to operate and is not entirely satisfactory in its effectiveness.